This invention is directed to treatment of industrial waste waters containing hexavalent chromium in the form of chromic acid, metallic chromate salts, etc. Such chrome materials can no longer be discharged directly into sewers, rivers or streams due to various legal requirements and regulations imposed by environmental authorities. Also, chromium is an expensive metal and it is economically desirable to recover the chromium values contained in the plating and rinse tanks. A process for removing the chromium from waste water and recovering it economically and efficiently has long been desired.
Prior art methods for removing chromium from industrial waste waters containing hexavalent chromium involve reducing the pH of the chromium waste water to a highly acid condition, e.g., 2.0 to 3.0, using a strong acid such as sulfuric acid or sodium bisulfite or sulfur dioxide (by adding it to a system through an SO.sub.2 feeder), while agitating the treated solution by mechanical means, or by air. Then caustic soda, lime or another basic substance is added to elevate the pH to a neutral or slightly basic condition, e.g., approximately 7.0 to 8.0, which then effects precipitation of the chrome in the trivalent form. The thus precipitated solution is then permitted to settle for four to six hours, after which water is pumped to the sewer and sludge is discarded to landfill. In many of these prior procedures, the settlement of the precipitate is accomplished with one or more clarifiers which serve to reduce the volume of the sludge for discarding at a landfill. In recent years it has become increasingly difficult to discard this sludge to landfills because the chrome material present therein tends to leach out and thus find its way back into the soil and into the streams, causing water pollution. Moreover, in addition to great difficulty in meeting current environmental protection standards, these methods are costly and comparatively inefficient.
Another prior process involves the use of ion exchange materials to remove chrome. The ion exchange procedures are extremely costly and not practical due to the large amounts of water required in such processes.
U.S. Pat. No. 3,371,034 to Richards, illustrates a direct precipitation process utilizing great quantities of barium carbonate and aqueous solutions acidified with strong acids, such as nitric or hydrochloric acid, or their salts. Such procedures encountered difficulties in separation of the chromium solids which are precipitated from the liquid waste media, thereby necessitating the use of one or more settling tanks which can be required to handle the excessive amounts of sludge produced. Moreover, the requirement for large amounts of barium carbonate increases the amount of sludge generated.
Another prior process, set forth in U.S. Pat. No. 3,869,386 to Izdebski, provides for direct removal of hexavalent chromium by adding aqueous barium acetate thereto. Izdebski states that this precipitation can be accomplished from neutral or slightly acid solutions in cases of precipitating both chromic acid and dichromates. Disadvantages encountered with the Izdebski use of barium acetate only are that the acetate becomes more than is needed, thus having acetates in the water effluent and having to add more barium acetate to get the desire barium radical that is needed to form barium chromate. At this point barium chromate is discarded. No reuse of the chrome is contemplated by Izdebski.
Accordingly, the need exists for a total recovery system which is economical and efficient, which permits reuse of both the recovered chromium and clarified water, and which may be installed as a single treatment unit in present plating systems.